PREPRINT
5
O.G.BUZYKIN, S.V.IVANOV, A.A.IONIN,
A.A.KOTKOV, A.YU.KOZLOV
SPECTROSCOPIC DETECTION
OF SULFUR OXIDESIN
THE AIRCRAFT WAKE
MOSCOW 2004
SPECTROSCOPIC DETECTION OF SULFUR OXIDES
IN THE AIRCRAFT WAKE
O.G.Buzykin*, S.V.Ivanov**, A.A.Ionin, A.A.Kotkov, A.Yu.Kozlov
P.N.Lebedev Physics Institute of Russian Academy of Sciences, 53 Leninsky prospect,
119991, Moscow, Russia, phone/fax: (095) 132-0425, e-mail: aion@mail1.lebedev.ru
*Central Aerohydrodynamic Institute (TsAGI), 140160, Zhukovsky-3, Moscow Region, Russia,
phone: (095) 556-4807, e-mail: bog@dept.aerocentr.msk.su
**Institute on Laser Information Technologies of Russian Academy of Sciences (ILIT RAS),
2 Pionerskaya, 142190, Troitsk, Moscow Region, Russia,
phone: (095) 334-0992, e-mail: s.v.ivanov@mtu-net.ru; ivanov@tsagi.ru
Abstract
Absorption/emission spectral regions of SO, SO2, SO3, S2O and HSO are analyzed for the
range from UV (λ ≥ 0.2 µm) to IR (λ < 30 µm) and are compared with atmospheric transmission
spectrum. It is shown that many vibrational bands of considered compounds fall into atmospheric
transmission windows. For vibrational bands of SO, SO2, SO3, S2O and HSO molecules there are
some gases which hinder the absorption diagnostics of indicated compounds. These interfering
gases are natural components of atmospheric air as well as specific gases of aircraft engine
exhaust. It is found that the least influence of the interference takes place in 2400-2700 cm-1 IR
region. Spectroscopic techniques applied for the detection of aircraft engine exhaust compounds
are briefly reviewed attending much consideration to SO2. IR absorption spectra of SO2 and other
gases are calculated for the conditions of aircraft engine nozzle exit. Narrow spectral intervals
suitable for SO2 detection in a hot flow are determined. The analysis is made for the detection
capabilities of CO2 lasers (including isotope CO2 lasers) and CO lasers (both fundamental band
and first-overtone ones) as applied for SO2 detection in aircraft engine exhaust.
СПЕКТРОСКОПИЧЕСКОЕ ДЕТЕКТИРОВАНИЕ
ОКИСЛОВ СЕРЫ В САМОЛЕТНОМ СЛЕДЕ
О.Г. Бузыкин, С.В. Иванов, А.А. Ионин, А.Ю. Козлов, А.А. Котков
Проанализированы спектры поглощения/излучения молекул SO, SO2, SO3, S2O и
HSO в области от УФ (λ ≥ 0.2 мкм) до ИК (λ < 30 мкм) и проведено их сравнение со
спектрами атмосферного пропускания. Показано, что многие колебательные полосы
рассмотренных соединений попадают в атмосферные окна прозрачности. Для
колебательных полос молекул SO, SO2, SO3, S2O и HSO определены газы, мешающие
диагностике их поглощения. Эти газы являются как естественными компонентами
атмосферного воздуха, так и специфическими газами выхлопа авиационного двигателя.
Обнаружено, что наименьшее влияние этих газов имеет место в ИК диапазоне частот 24002700 см-1. Кратко рассмотрены спектроскопические методы детектирования веществ в
выхлопе авиационного двигателя, при этом наибольшее внимание уделено SO2. Рассчитаны
ИК спектры поглощения SO2 и других газов для условий на срезе сопла двигателя самолета.
Определены узкие спектральные интервалы, пригодные для обнаружения SO2 в горячей
струе. Проанализированы возможности CO2 лазеров (включая лазеры на изотопах СО2) и
CO лазеров (действующих на фундаментальных колебательных переходах и на переходах
первого колебательного обертона) применительно к обнаружению SO2 в выхлопах
авиационного двигателя.
2
Contents
1. Introduction
4
2. Spectroscopic properties of some sulfur oxides
5
3. Absorption bands of sulfur oxides and atmospheric transmission spectra
7
4. Brief review of spectroscopic techniques for SOx detection in a hot flow
12
Spectroscopic detection of gases in aircraft wake
5. Calculation of SO2 IR absorption spectrum for aircraft exhaust flow
12
16
6. Analysis of diagnostic possibilities of CO2 and CO lasers for SO2
detection in aircraft exhaust jet
20
7. Conclusions
23
8. Acknowledgment
25
9. References
25
Tables
28
3
1. Introduction
Sulfur compounds (mainly SO, SO2 and SO3) play a key role in chemical
reactions taking place in aircraft vortex wake [1]. Concentrations of these
compounds at the exit of engine nozzle and in the near wake region govern
essentially the further evolution of reactions in other wake areas. The absence of
reliable and detailed information on the amount of sulfur compounds in different
sections of wake restrains the solution of a number of practical problems related to
the ecological impact of aviation on the Earth atmosphere. It is clear that the
measurement of concentrations in the immediate vicinity of an aircraft engine is
possible only remotely, i.e. by spectroscopic methods.
Spectroscopic methods are of growing interest today due to recent
development of powerful lasers that are tunable over large wavelength ranges, which
permits the detection of numerous species at low concentrations. In contrast to nonspectroscopic schemes (gas chromatography, etc.) these methods exhibit some
unique advantages. They allow non-intrusive, continuous and simultaneous
detection of many gases. In some cases they allow three-dimensional space mapping
of pollutant concentrations.
Determination of concentrations of sulfur compounds in aircraft wake by
spectroscopic methods is, as a matter of fact, the problem of multicomponent
mixture analysis. The measurements are rather complicated because of low
concentrations of detected gases. To solve such a problem it is necessary to know
the absorption (or Raman scattering) cross-sections of all the components of gas
mixture at given frequency. The accuracy of concentrations determination depends
on the precision of this information, as well as on the choice of sounding
frequencies. Sounding frequencies must: 1) coincide with strong lines of detected
gas minimizing the overlap with lines of other substances; 2) fall into transmission
windows (or microwindows) of the atmosphere.
4
Detailed absorption spectra of substances are usually measured directly or
calculated by means of special computer codes (e.g., FASCODE, LOWTRAN,
MODTRAN) using spectroscopic databases (e.g., HITRAN, GEISA). In the
calculations the positions and intensities of multiple lines (electronic and
vibrational-rotational) are taken into account as well as their spectral contours,
which depend on temperature and pressure. Despite great amount of researches on
sulfur compounds, it is not enough detailed spectroscopic information for these
molecules. For example, HITRAN database contains the information only on some
bands of SO2 and H2S.
We consider here the following sulfur compounds: SO, SO2, SO3, S2O and
HSO. The main spectroscopic properties of indicated compounds are reviewed
below. Spectral regions of radiation absorption/emission are analyzed for each
substance, and interfering gases of atmospheric air and aircraft wake are identified.
Also, the spectroscopic schemes applied to detection of aircraft engine exhaust gases
are briefly reviewed. The much consideration is given to SO2 detection. IR spectra
of SO2 and other exhaust gases are calculated for the engine nozzle exit conditions
of Boeing 707 aircraft. The analysis is made for detection capabilities of CO2 and
CO lasers as applied for SO2 detection in aircraft exhaust flow.
2. Spectroscopic properties of some sulfur oxides
SO (sulfur monoxide). Ground electronic state of this radical is X3∑-. S-O bond
energy is D0(S-O)= 5.34±0.02 eV. The electronic transitions B3∑- − X3∑- and А3П −
X3∑- were observed in the UV regions 1900-2600 and 2400-2600 Å respectively.
SO(X3∑-) radical is formed as a primary product in SO2 photolysis in the
wavelength region below 2190 Å [2]. In the ground electronic state of
32 16
S O, its
fundamental band is centered at ν01= ωe - 2ωexe= 1137.96 см-1. Herewith ωe=
1149.22 см-1, ωexe = 5.63 см-1 [3]. More detailed information on electronic and
vibrational- rotational spectra of SO is contained, e.g., in [4-10].
5
~
SO2 (sulfur dioxide). Ground electronic state is X 1 A1 , the angle between O-S-O
bonds is 119.5°. SO2 is bent triatomic molecule with C2v symmetry, showing slightly
asymmetric top rotational spectrum properties [2, 11]. Bond energy D0(ОS-O) is
5.65±0.01 eV. Sulfur dioxide has complex absorption spectra in near and vacuum
UV regions. There exist three main regions of absorption in UV: very weak
absorption in the range of 3400-3900 Å, weak absorption in 2600-3400 Å, strong
absorption in 1800-2350 Å [2]. SO2 dissociation to SO+O begins under the action of
light with λ< 2190 Å. At the wavelengths longer than 2190 Å the light causes strong
fluorescence and phosphorescence of SO2. Detailed structure of SO2 electronic
bands and relevant absorption cross sections were studied in [12]. SO2 molecule has
intense vibrational and rotational spectra, which are currently sufficiently well
studied. Information on high-resolution spectra of many SO2 bands is contained in
HITRAN database [13] (see Table 2).
SO3 (sulfur trioxide). Ground electronic state of SO3 has a plane structure with D3h
symmetry; O-SO2 bond energy is D0(O-SO2)=3.55±0.01 eV. Absorption spectrum of
sulfur trioxide begins near 3100 Å and consists of weak diffuse bands,
superimposing the continuum [2]. In [14, 15] SO3 UV absorption spectrum was
studied in detail with the indication of cross sections. In the Table 4 the positions of
main vibrational bands of SO3 are presented. Rotational Raman spectrum of SO3
was obtained in [18]. It is possible to use Raman band at 1068 cm-1 for SO3
determination in SO2 by measurement of intensity of this band relative to SO2 band
at 1151 cm-1 [2].
S2O (disulfur oxide). Disulfur oxide is bent triatomic molecule having Cs symmetry
(S=S=O) and, consequently, three nondegenerate vibrational modes are expected:
ν1(S-O stretch)= 1166.45 cm-1, ν2(bending)= 382 cm -1, ν3(S-S stretch)= 679.14 cm-1.
There is some overlap of ν1 band of S2O near 1166 см-1 and ν1 band of SO2 band
~
near 1152 см-1. In the ground electronic state X 1 A' structural parameters are rSO=
6
1.4594 Å, rSS= 1.8845 Å, ∠ SSO=118.08°. Intense richly structured absorption and
~
~
emission spectra in ∼ 3400-2500 Å are studied and assigned as C 1A'− X 1 A' (π*−π)
electronic transition [19]. High- resolution spectra of S2O bands were studied, e.g.,
in [20, 21].
~
HSO. Ground electronic state of HSO radical is X 2 A' ' , the angle between Н-S-N
bonds is 102°; bond energies are D0(H-SO) ≈ 1.6 eV and D0(HS-O)=3.4 eV [2].
Chemiluminescence observed in the region of 5200-9600 Å in the flow containing
O-H2S-O3 mixture, was referred in [22] to 2A'−2A'' transition of HSO radical.
Excited
electronic
state
2
А',
probably,
is
formed
in
the
reaction
SH+O3→HSO(2A')+O2. Fundamental vibrational frequencies of HSO molecule in
electronic states 2A'' and 2A' are presented in Table 3.
3. Absorption bands of sulfur oxides and atmospheric transmission spectra
Fig. 1 shows transmission spectra of standard atmosphere in a wide
wavelength range (from IR to UV). Spectra are calculated using MODTRAN code.
It is seen from this figure that abrupt transmission decrease at the beginning of UV
region is caused by strong absorption of radiation by ozone (Hartley bands), oxygen
(Schumann-Runge system) and by Rayleigh (molecular) scattering of radiation. In
Fig. 2 the transmission in IR bands of main atmospheric gases is plotted. In Fig. 3
the wavelength regions of electronic absorption bands of SO, SO2, SO3, S2O and
HSO are shown. Fig. 5 shows the positions of IR absorption bands centers of these
molecules. Fig. 4 contains more detailed information on absorption cross-sections of
SO2, SO3, COS and CS2 in UV region. From all these figures it is possible to
conclude that UV absorption diagnostics of considered sulfur compounds is possible
only at small distances. On the other hand, in visible, IR and microwave ranges there
is a number of transmission windows (as well as microwindows) where the
attenuation of radiation in the atmosphere is low and, hence, diagnostics is possible
using large distances. From the standpoints of sulfur compounds detection the
7
following windows are interesting: 500-600 cm-1 (for SO2 , SO3), 1150-1200 cm-1
(for SO, SO2 , S2O), 2450-2800 cm-1 (for SO2 , SO3, HSO).
1,0
O2
Total
Transmission
0,8
Rayleigh O
3
scattering
0,6
0,4
0,2
0,0
0
10000
20000
30000
40000
50000
-1
Wavenumber, cm
Fig.1. Transmission spectra of standard 1976 US atmosphere from far IR to UV. MODTRAN
calculations. Horizontal path L=5 km at altitude H=12.2 km. No aerosols included.
Spectral resolution is 25 cm-1.
1,0
5
4
Transmission
0,8
3
0,6
1
1
4
0,2
2
0
1000
2
2000
5
1-H2O
2-CO2
3-O3
4-N2O
5-O2
2
3000
1
2
1
0,4
0,0
1
4000
5000
6000
7000
8000
-1
Wavenumber, cm
Fig.2. Infrared transmission spectra of standard 1976 US atmosphere. MODTRAN calculations.
Horizontal path L=5 km at altitude H=12.2 km. No aerosols included. Spectral resolution is
25 cm-1.
8
SO
1,0
Transmission
0,8
SO3
1
0,6
0,4
HSO
SO2
S2O
Standard atmosphere, H=12.2 km
1-L=0.2 km
2-L=1 km
3-L=5 km
2
0,2
3
0,0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1,0
Wavelength, µm
Fig.3. Regions of electronic absorption bands of SO, SO2, SO3, S2O and HSO (see the text).
Transmission spectrum of standard aerosolless atmosphere is shown (MODTRAN
calculations). Horizontal path L (indicated near the curves) at altitude H=12.2 km.
Let define the gases, which can hinder absorption diagnostics of sulfur
compounds in aircraft wake. Amongst the atmospheric gases we consider molecules
contained in HITRAN-96 spectroscopic database: H2O, CO2, O3, N2O, CO, CH4, O2,
NO, SO2, NO2, NH3, HNO3, OH, HF, HCl, HBr, HI, ClO, OCS, H2CO, HOCl, N2,
HCN, CH3Cl, H2O2, C2H2, C2H6, PH3, COF2, SF6, H2S, HCOOH. Many of these
gases are presented in significant amount (above atmospheric) in exhausts of aircraft
engines (e.g., H2O, CO2, CO, NO, NO2, HNO3, OH, H2CO, H2O2, C2H2). Among
volatile organic compounds the following gases are additionally present in the
exhausts: ethene (C2H4), propene (C3H6), acetaldehyde (C2H4O), acrolein (C3H4O),
benzene (C6H6), toluene (C7H8), etc. Table 5 shows the information on interfering
gases, which hinder detection of sulfur oxides in IR region within indicated spectral
intervals (±100 см-1 offset is taken around each central frequency). Fig. 6 shows IR
absorption spectra of some organic substances related to aircraft engine exhausts as
well as the positions of IR bands of SO, SO2, SO3, S2O, HSO. From Table 5 and Fig.
9
6 it follows that minimum influence of interfering gases is observed in the IR region
of 2400-2700 cm-1. It should be pointed out that this spectral region overlaps with
lasing range of first-overtone CO laser [52].
-16
10
SO2 213 K
SO2 213 K
-17
10
Absorption cross section, cm
2
Absorption cross section, cm
2
10
-17
-18
10
CS2 293 K
-19
10
-20
10
SO3
room temp.
COS
294 K
-21
10
-22
-18
10
COS 294 K
(circles)
-19
10
SO3
room temp.
(squares)
10
-20
160
200
240
280
10
320
210
220
230
240
Wavelength, nm
Wavelength, nm
Fig.4. Absorption cross-sections of SO2 [12], SO3 [15], COS and CS2 [14] in UV region. Right
figure shows in detail the spectrum in the region of 208-240 nm.
1,0
4 3 23 4
5 5 124
3 2 5
23
3
Transmission
0,8
0,6
0,4
1-SO
2-SO2
3-SO3
4-S2O
5-HSO
0,2
0,0
500
1000
1500
2000
2500
-1
Wavenumber, cm
Fig.5. Position of IR bands for SO, SO2, SO3, S2O and HSO. Transmission spectrum of standard
US 1976 atmosphere is shown. No aerosols included. H=12.2 km, horizontal path L=5 km.
10
Absorbance, rel.un.
1,0
4 323 4
55124 23
3
5
3
Propene
C3H6
(solid)
Acrolein
C3H4O
(squares)
0,8
0,6
1-SO
2-SO2
3-SO3
4-S2O
5-HSO
0,4
0,2
0,0
2
500
1000
1500
2000
2500
3000
-1
Wavenumber, cm
Absorbance, rel.un.
1,0
4
323 4
55
124 23
0,6
2
5
3
Benzene
C6H6
(solid)
Toluene
C7H8
(circles)
0,4
0,2
0,0
3
1-SO
2-SO2
3-SO3
4-S2O
5-HSO
0,8
(a)
500
1000
1500
2000
2500
-1
Wavenumber, cm
3000
(b)
Fig.6. Infrared absorption spectra of some organic compounds related to aircraft engine exhaust.
Each spectrum is normalized to its maximum. Position of IR bands of SO, SO2, SO3, S2O
and HSO are indicated.
11
4. Brief review of spectroscopic techniques for SOx detection in a hot flow
Air pollution monitoring by spectroscopic techniques has been reviewed in
many monographs and articles, e.g., [23-31]. The physical phenomena used in
spectroscopic detection are as follows:
a) Raman scattering. Advantages: laser emission at a single wavelength is sufficient
for detection of many gases. Shortcomings: small scattering cross-sections; possible
interference with fluorescence; use of a high-frequency laser is desirable (UV,
visible).
b) Laser-induced fluorescence (LIF). Advantages: especially suitable for detecting
of atoms and radicals; somewhat larger scattering cross-sections compared to those
for Raman scattering. Shortcomings: multi-line emission quenched by collisions.
c) Absorption processes. Advantages: very large cross-sections (they are from six to
eight orders of magnitude larger than those for Raman scattering).
Today the most widely used spectroscopic detection schemes for air pollution
monitoring are based on absorption processes having the largest cross sections. The
most informative in the sense of spectroscopic absorption is IR range of spectrum,
which contains fundamental absorption bands, their overtones and combination
vibration-rotational bands of numerous gases of natural and anthropogenic origin.
This spectral region has an ensemble of very intensive and narrow lines of absorbing
components. Besides this, in IR a number of laser sources are successfully produced,
having discrete as well as continuous tuning of radiation frequency.
Spectroscopic detection of gases in aircraft wake
Presently, the monitoring of gas flows is conducted by different spectroscopic
methods. It is necessary, however, to notice that the words "diagnostics" or
"sounding" of the object are often treated in different manner. It is, for instance,
visualization, determination of vortex structure, temperature, concentrations of gases
and aerosols. The results of sounding can be qualitative as well as quantitative,
range-resolved as well as averaged over path length.
12
The following methods are presently used for the diagnostics of
concentrations of gases in exhaust flows:
- laser long-path absorption spectroscopy in visible and UV [32];
- IR tunable diode laser spectroscopy [33, 34];
- Fourier transform IR (FTIR) spectroscopy [35-37];
- laser heterodyne spectroscopy [24];
- laser-induced fluorescence [38];
- resonance holographic interferometry [39]
- Raman spectroscopy [24].
Emphasize that none of these methods is absolutely universal and free from
the shortcomings. Further we consider only some of these methods.
Fourier transform spectrometers allow to get the spectra from the
interferograms and then, by using the method of least square fitting of measured
spectra to calculated ones, extract the concentrations of gases in the mixture
considered. The "weak spot" of Fourier spectrometers is their strong sensitivity to
different mechanical influences and relatively large time required for the writing of
interferograms. However, recently the advanced Fourier IR spectrometer MIROR
(Michelson Interferometer with Rotating Retroreflector) was designed [36]. MIROR
possesses the increased stability; it is specially intended for the in-flight
measurements of aircraft engine emission indexes. However, its spectral resolution
(~0.12 cm-1) is not high, and, as a consequence, the sensitivity of measurements is
low and the variety of detecting molecules is limited by the gases which
concentrations in the flow is great (H2O, CO2, CO, NO). Ground-based laboratory
measurements of flow temperature and the concentrations of main exhaust gases of
aircraft engine (H2O, CO2, N2O, CO, CH4, NO, NO2, SO2, H2CO) were made using
Fourier transform IR spectrometer of greater, than MIROR, spectral resolution equal
to 0.06 cm-1 [35, 37]. The evaluated accuracy of the system was 30%. Even in this
case of high-resolution instrument, not all the gases could be measured (see Table
13
6). For example, it was impossible to detect the gases if their detection limits are
greater than the amount in the medium. These are such molecules as N2O, SO2,
H2CO, CH4. The authors [35,37] fairly noted the interfering role of spectral lines of
many gases of a mixture in determination of given gas concentration. Note that this
problem is well known in spectroscopic gas analysis.
Diagnostics of SO2 by means of diode lasers. Several lines of ν1 SO2 band were
registered in [40] by means of the Pb1-xSnx (x=0.07) diode laser operating in the
vicinity of λ=8.8 µm using the method of first derivative. The measurements were
conducted in the sample of air containing 670 ppmV SO2 sealed to the pressure of
5 Torr (path length was 7.3 m). The selectivity of measurements was extremely high
because of low pressure of the sample. Note that in experiment there the lines of
moderate intensity of ν1 band of SO2 were used . If one select the other stronger
lines, it is possible to enhance the signal approximately by the factor of 20. In the
book of Hinkley [24] the example is also cited related to SO2 measurements in situ
by means of IR diode laser spectroscopy. The λ=8.8 µm radiation of the Pb1-xSnx
(x=0.08) diode laser was directed across the pipe of the 5 m diameter to the receiver.
The transmission was only 10% because of burned coal particles. However, they
succeeded in defining real-time SO2 concentration under the acceptable signal-tonoise ratio. In [34] mid-IR led-salt diode laser and multipass cell were used to
measure a few ppmV SO2 concentrations in aircraft engine exhaust in an altitude test
chamber. Also, the possibility was demonstrated of similar sensitivity for SO3.
SO2 detection by the heterodyne method [24]. In the Hinkley's laboratory SO2
detection was made by the heterodyne method using the commercial CO2 laser with
12
C18O2 rare isotope active medium. The laser operating on this rare isotope radiates
shorter wavelengths than
12
C16O2 laser. The lines 9R(40) (ν=1107.9499 cm-1) and
9R(42) (ν=1108.9246 cm-1) were used. It was noted that the sensitivity of
heterodyne method in SO2 detection in the vicinity of λ ~ 9 µm turns out to be
higher at high temperatures than at 300 K. For example, at 400 K the sensitivity was
14
2.5 times higher than at 300 K. It was noted also that it is quite possible to reach the
limiting sensitivity of 10 ppmV at 400 K using the path length of 1 m. The
contribution of water vapor into absorption of radiation at each frequency must be
determined in separate measurements on additional wavelengths.
SO2 detection by means of spontaneous Raman scattering. Diagnostic methods
based on Raman scattering (RS) possess some unique advantages as compared to
absorption spectroscopy. The methods of laser remote sensing based on RS are very
promising since they provide the possibility of atmosphere sensing from one point.
The effect of RS itself does not depend on wavelength as well as on quenching
collisions being instant. The important advantage of RS based scheme is that it does
not require the selective excitation of different gases by separate frequencies because
the backscattered signal contains the spectra of all RS active air components with
the intensities of lines being proportional to concentrations. At the same time the
cross-sections of spontaneous RS are typically less than those of Rayleigh scattering
by approximately three orders of magnitude. For example, SO2 differential section
dσ/dΩ is approximately equal to 10-29 cm2/sr (nonresonant RS) and 10-26 cm2/sr
(resonant RS). As a result of careful measurements of RS differential cross section
 dσ 
of Q-branch of nitrogen there was obtained the following weighted


 dΩ  Q , N 2
 dσ 
average value 
= (5.05 ± 0.1) ⋅ 10 −48 ⋅ (ν 0 − 2331) 4 , cm6/sr, where ν0 is a

 dΩ  Q , N 2
wavenumber of exciting radiation (cm-1). Differential cross-sections of other gases
can
be
calculated
using
the
following
expression
(ν 0 − ν j ) 4
 dσ 
=
⋅
⋅ ∑ j , where νj is RS band center of the component

dΩ  dΩ  Q , N 2 (ν 0 − 2331) 4
dσ j
j; Σ j is relative (as compared to N2) normalized total (i.e., for the whole band)
differential cross section of RS for the component j (see Table 7).
15
In [41] by means of mobile installation the diagnostics of H2O, CO2, SO2 and
kerosene vapor was made with the spatial resolution of ~10 m at the day time. The
estimates of the contents of SO2 (300 ppmV) and kerosene (17 ppmV) molecules
prove the service ability of RS method for the remote monitoring of the air pollution
sources even at a day time. In [42] the results of field experiments on the remote
determination of composition and temperature of combustion products of aircraft
gas turbine engine were described. Nitrogen laser (λ=337.1 nm) was used at the
pulse repetition rate of 500 Hz. The analysis of H. Inaba for typical RS LIDAR
systems made in the book [24] shows that carefully developed RS LIDAR scheme
will have the sensitivity allowing to detect the atmospheric pollution having the
concentrations of several ppmV at distances of several hundred meters. Such
sensitivity being, certainly, insufficient for the measurement of pollution in usual air,
is completely suitable for monitoring the content of different chemical substances in
stationary sources: injections of smoke-stacks, exhausts of car and aircraft engines.
5. Calculation of SO2 IR absorption spectrum for aircraft exhaust flow
In order to study the possibilities of SO2 detection by means of IR absorption we
calculated IR spectrum of jet exhaust gases of Boeing 707 aircraft as well as of
surrounding atmosphere at the cruising altitude of Н=12.2 km. The calculations
were made for typical conditions of the US standard atmosphere and the engine exit
plane (see Table 8). The results of calculations are presented in Fig. 7 and 8. The
absorption lines of hot SO2 in IR are overlapped mainly by H2O, CO2, N2O and CH4
gases (the order of gases reflects the degree of their interference). In Fig. 7 the
absorption spectrum of hot SO2 and total spectrum of all the other exhaust gases are
shown. Also, the absorption spectrum of ambient (cold) atmosphere is shown (the
same gases as in exhaust plus ozone; certainly, with mole fractions in ambient air).
Atmospheric mole fractions are taken from [43].
16
4
Absorption coefficient, km
-1
10
2
10
0
10
-2
10
Symbols-SO2
Line-hot wake gases total
Gray-cold ambient atmosphere
-4
10
-6
10
450
500
550
600
-1
Wavenumber, cm
4
Absorption coefficient, km
-1
10
2
10
0
10
-2
10
-4
10
1050
1100
1150
1200
1250
-1
Wavenumber, cm
Fig.7. Absorption spectra of hot SO2 and total hot wake gases in B707 near wake. Ambient total
absorption is also shown (the same gases+ozone). Altitude H=12.2 km, P=0.1715 atm.
Nozzle exit plane conditions. T=604.7 K. Mole fractions are taken from [1] and [43].
17
5
Absorption coefficient, km
-1
10
3
10
1
10
-1
10
-3
10
1320
1340
1360
1380
-1
Wavenumber, cm
1
10
Absorption coefficient, km
-1
0
10
-1
10
-2
10
-3
10
-4
10
2470
2480
2490
2500
2510
2520
-1
Wavenumber, cm
Fig.7 (continued). Absorption spectra of hot SO2 and total hot wake gases in B707 near wake.
Ambient total absorption is also shown (the same gases+ozone). Altitude H=12.2 km,
P=0.1715 atm. Nozzle exit plane conditions. T=604.7 K. Mole fractions are taken from [1]
and [43].
The difference of absorption spectra of SO2 and all other (background) gases of
hot jet is presented in Fig. 8. These data allow us to select the spectral intervals,
which are suitable for SO2 diagnostics in a hot exhaust of aircraft.
18
0
αSO - αbackground, km
2
-1
10
-1
10
-2
10
-3
10
1100
1120
1140
1160
1180
1200
1220
1240
-1
Wavenumber, cm
1
10
αSO - αbackground, km
2
-1
0
10
-1
10
-2
10
-3
10
1320
1330
1340
1350
1360
1370
-1
Wavenumber, cm
Fig. 8 (see also below).
19
1380
1390
-1
αSO - αbackground, km
2
-1
10
-2
10
-3
10
2460
2470
2480
2490
2500
2510
2520
2530
-1
Wavenumber, cm
Fig.8.(continued). Difference of absorption spectra of hot SO2 and total hot wake gases in B707
aircraft near wake. Altitude H=12.2 km, P=0.1715 atm. Nozzle exit plane conditions [1].
T=604.7 K.
The most contrast of SO2 with respect to absorption of other gases is observed on
the following frequencies: 1380.88 cm-1 (∆α=3.62 km-1), 1348.23 cm-1 (∆α=2.58
km-1), 1376.28 cm-1 (∆α=2.36 km-1), 1383.82 cm-1 (∆α=2.08 km-1), 1332.88 cm-1
(∆α=1.93 km-1). Note that at frequency 1380.88 cm-1 absorption coefficient of SO2
reaches its maximum equal to αSO2=3.945 km-1.
6. Analysis of diagnostic possibilities of CO2 and CO lasers for SO2 detection in
aircraft exhaust jet
СО2 and СO lasers [44, 45] being now one of the most powerful IR lasers are
widely used in spectroscopic diagnostics of various media. CO laser operating in
fundamental and first-overtone bands looks like an attractive device for remote
spectroscopic gas analysis [46-50]. Our goal was to evaluate the diagnostic
possibilities of indicated lasers for SO2 detection for the conditions of aircraft
exhaust jet. The calculations were performed using the code ANLINES [51] and
20
HITRAN-96 database [13]. The following extended sets of emission frequencies of
lasers were used. CO2 laser: main isotope 626 (i.e.
16
O12C16O) and isotope
modifications 627, 628, 636, 828; total 713 transitions from 857.94 to 1116.05 cm-1.
The positions of CO2 laser lines were taken from the book of Witteman [44].
Fundamental band CO laser (main isotope 12C16O; 726 transitions from 1162.42 to
2022.72 cm-1). First-overtone CO laser (main isotope
12
C16O, 726 transitions from
2364.93 to 4034.06 cm-1). The positions of 726 transitions V=4-36, J=4-25 of
fundamental band СО laser and first-overtone СО laser [52] were calculated using
spectroscopic constants of 12C16O molecule [53].
The analyzed gas mixture consisted of nine IR active gases: H2O, CO2, N2O,
CO, CH4, NO, SO2, NO2, OH. The pressure-induced absorption of N2 and O2 was
considered as a continuum. Besides this, N2 and O2 gases served as broadening
gases. Analytical lines were selected using the following criterion: 1) nearly the
exact resonance with of laser frequency with molecular absorption line (within 2
halfwidths); 2) the contribution of given molecular line into absorption crosssection of the gas considered must be greater than 10-22 cm2. Using this criterion
there were selected 82 lines of CO2 laser, 366 lines of fundamental band CO laser
and 244 lines of first-overtone CO laser. These three sets of analytical frequencies
were then examined for sensitivity and selectivity in determination of concentrations
were calculated
of gases of the mixture. Minimum detectable concentrations c min
j
assuming moderate value of minimum detectable absorption coefficient α min = 10-6
cm-1. Note that typical α min value for CO2 laser photoacoustic spectroscopy [8] is
less than 10-7 cm-1.
The detection sensitivity of a substance j is related with the strength of its absorption
and
usually
is
defined
by
minimum
detectable
concentration
c min
j :
c min
= (1 / N ) ⋅ min (α min / σ ij ) , where α min is the minimum detectable absorption
j
i
21
coefficient defined by the method of measurement and apparatus, σ ij is the
absorption cross-section of substance j at laser transition i, N is total concentration of
molecules in mixture. Note that c min
is the exact measure of minimum detectable
j
concentration only for interference-free conditions, i.e., when spectra of other
substances of the mixture do not overlap the spectrum of gas j at laser frequency i
(this is exactly the case when the gas j is a single absorber in the mixture.
For the determination of the concentration of substance j on corresponding
laser frequency j the possible influence of substance k is described by means of cross
sensitivity Q jk = σ jk σ jj . The uncertainty ∆ck of determination of concentration
ck of the substance k increases the uncertainty ∆c j of measured concentration c j of
the substance j as follows: ∆c j ≥ Q jk ∆ck . It is clear from this relation that to
enhance the accuracy of gas analysis of multicomponent mixture it is necessary to
select those laser frequencies at which the values of cross sensitivities Q jk (j ≠ k) are
small (or reduce ∆ck , that is not always possible).
The results of the calculation of cross sensitivity Q jk for selected analytical
frequencies of СО2 laser, fundamental band CO and first-overtone CO lasers are
presented in Tables 9, 11, 13. Minimum detectable concentrations c min
j , the estimate
of total interference of other gases
∑ Q jk
and absorption coefficients for analytical
j ≠k
frequencies of СО2, fundamental band CO and first-overtone CO lasers are given in
Tables 10, 12, 14.
It is follows from the listed data that:
1) minimum detectable concentrations of SO2 which can be measured by means of
СО2 lasers, fundamental band CO laser and first-overtone CO laser are respectively
16.7, 0.38 and 23.8 ppmV, i.e. the best sensitivity of detection can be reached with
using fundamental band CO laser;
22
2) when using the II 828 R(54) transition of СО2 laser for SO2 diagnostics the
noticeable interference of N2O spectrum occurs. In this case Q jk =0.28. Note that
this laser frequency falls into solely "clear" spectral interval (absorption coefficient
is only 0.07 km-1);
3) when detecting SO2 using 28-27 P(23) line of fundamental band CO laser the
spectral interference of other gases (amongst considered) is negligible. At the same
time the total absorption is noticeable (α=4.4 km-1). It is mainly caused by water
vapor which concentration in aircraft wake is great;
4) when detecting SO2 with using 35-33 P(22) line of first-overtone CO laser the
spectral interference of other gases (amongst considered) is also low (low influence
results only from N2O, Q jk =0.04). The total absorption at this frequency is low (α=
0.03 km-1) being less than that for СО2 laser.
7. Conclusions
1.
Spectral regions of absorption/emission of sulfur oxide molecules (SO, SO2,
SO3, S2O and HSO) are analyzed in a wide wavelength range from UV (λ ≥ 0.2 µm)
to IR (λ < 30 µm
). In the IR range these regions correspond to vibrational
transitions, while in visible and UV ranges to electronic transitions of indicated
molecules.
2.
Atmospheric transmission spectra are calculated and analyzed in a wide
wavelength range (from IR to UV). It is shown that absorption diagnostics of sulfur
oxides in the UV range is possible only at small distances because of strong
absorption of radiation by ozone, oxygen and Rayleigh scattering. On the other
hand, in visible, IR and microwave regions there are some transmission windows (as
well as microwindows), where attenuation of radiation is low, and so long-path
diagnostics is possible. This is particularly important for the external diagnostics of
aircraft wake (from the board of another air vehicle). It is found also that many
vibrational bands of sulfur compounds considered just fall into transmission
23
windows: 500-600 cm-1 (SO2 , SO3), 1150-1200 cm-1 (SO, SO2 , S2O), 24502800 cm--1 (SO2 , SO3, HSO).
3.
For vibrational bands of SO, SO2, SO3, S2O and HSO the gases are
determined which can hinder IR absorption diagnostics of indicated molecules.
These interfering gases are natural components of atmospheric air as well as specific
gases of engine exhaust. It is found that the least influence of interfering gases takes
place in 2400-2700 cm-1 region overlapping with lasing range of a first-overtone CO
laser.
4.
The analysis of traditional spectroscopic methods with reference to detection
of aircraft exhaust gases demonstrates that the measurement of SO2 concentration in
the near wake of aircraft (SO2 mole fraction is about 1-10 ppmV) is quite real to be
realized by various methods. These can be, e.g., long-path laser IR absorption
spectroscopy (including CO and CO2 lasers, diode spectroscopy, heterodyne
spectroscopy, etc.), as well as Raman scattering LIDAR method. LIDAR scheme
based on RS can have the sensitivity, allowing to detect the atmospheric pollution
having the concentrations of several ppmV at a distance of several hundred meters.
For the time being, Fourier transform spectroscopy does not seem to be able to
ensure acceptable sensitivity of SO2 detection in aircraft exhaust.
5.
The spectral intervals suitable for SO2 IR absorption detection in engine
exhaust hot flow are determined. If very sensitive detection of SO2 is important,
1330-1390 cm-1 interval is preferable for measurements. The most contrast of SO2
absorption with respect to absorption of other gases is observed on the following
frequencies:
1380.88 cm-1
(∆α=3.62 km-1),
1376.28 cm-1
(∆α=2.36 km-1),
1383.82 cm-1
1348.23 cm-1
(∆α=2.08 km-1),
(∆α=2.58 km-1),
1332.88 cm-1
(∆α=1.93 km-1). In the case of SO2 detection with enhanced selectivity the spectral
region of 2400-2700 cm-1 is preferable.
6.
The analysis is made for the detection capabilities of CO2 lasers (including
isotope CO2 lasers) and CO lasers (fundamental band and first-overtone ones) as
24
applied for SO2 detection in aircraft exhaust flow. Calculated minimum detectable
concentrations of SO2 were found to be 16.7, 0.38 and 23.8 ppmV for СО2 lasers,
fundamental band CO laser and first-overtone CO laser respectively. These
estimates were obtained in the suggestion that minimum detectable absorption
coefficient of apparatus is 10-6 cm-1. Thereby, the sensitivity of SO2 detection is the
best in the case of application of fundamental band CO laser. The spectral
interference of other gases (amongst considered) was found as a totally negligible
for determination of SO2 concentration with using selected analytical lines of
considered lasers.
8. Acknowledgment
This research was supported by the ISTC Projects 2249 and 2415P.
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Table 1. Electronic transitions of SO radical [2].
Electronic
state
X3∑a1 ∆
b1∑+
А3П0
B3∑-
Energy
Е0, eV
0
0.79
1.303
4.748
5.161
System
(emission region)
b→X, (9500-10900 Å)
А→Х (2400-2600 Å)
В→Х (1900-2600 Å)
Lifetime,
ns
17±3
Table 2. The most significant absorption bands of 32S16O2 in microwave and IR ([13]).
Band
V’-V’’
(000)-(000)
(010)-(000)
(100)-(000)
(001)-(000)
(111)-(010)
(101)-(000)
λ0,
µm
19.31
8.68
7.34
4.01
4.00
ν0,
cm-1
νmin-νmax ,
cm-1
Number of
lines
517.75
1151.7135
1362.0295
2492.4438
2499.8701
0-257
433-617
1047-1262
1316-1394
2463-2516
2463-2527
1883
3326
5812
2075
654
1883
Table 3. Fundamental vibrational frequencies of HSO molecule, cm-1 [22].
Band
ν1
ν2
ν3
Electronic state 2A'
Electronic state
X2A''
2570
1063
1013±5
2769
828
702±5
28
Integral
intensity,
cm/molec.
3.954⋅10-19
3.899⋅10-18
3.519⋅10-18
3.080⋅10-17
2.110⋅10-20
3.954⋅10-19
Table 4. Vibrational bands of SO3 [16, 17].
Band
ν1( a1 ' )
ν2( a2 ' ' )
ν3( e ' )
ν4( e ' )
ν1+ν3( e ' )
2ν3( e ' )
Band center, cm-1
1068
495
1391.5205
529
2443
2773
Note
Only Raman active
Table 5. Interfering gases which hinder absorption diagnostics of sulfur oxides in IR region.
Sulfur
oxide
Band
Range,
cm-1
SO
0-1
1137.96
±100
SO2
ν2
517.75
±100
1151.7135
±100
ν1
ν3
HSO
SO3
1362.0295
±100
ν1+ν2 2492.4438
+ν3-ν2 ±100
2499.87
ν1
±100
1063
ν2
±100
ν3
1013
±100
ν2
495
±100
1391.5205
±100
ν3
ν4
ν1+ν3
529
±100
2443
Interfering gases of atmospheric air
Other interfering
and engine exhaust (by HITRAN-92) organics contained in
engine exhaust
H2O, CO2, O3, N2O, CH4, SO2, NH3, Acrolein, benzene,
HNO3, HOCl, H2O2, C2H2, PH3,
toluene, ethene
COF2, H2S
H2O, CO2, O3, N2O, CH4, NO2, NH3, Acrolein, propene,
toluene
HNO3, HF, HCl, HCN
H2O, CO2, O3, N2O, CH4, NH3,
Acrolein, benzene,
HNO3, HOCl, H2O2, C2H2, PH3,
toluene, ethene
COF2, H2S
H2O, CO2, O3, N2O, CH4, O2, NH3,
Acrolein, benzene,
HNO3, OH, HOCl, HCN, H2O2,
propene, toluene,
C2H2, PH3, COF2, H2S
ethene
H2O, CO2, O3, N2O, CH4, OH, HCl,
HBr, HI, N2
H2O, CO2, O3, N2O, CH4, OH, HCl,
HBr, HI, N2
H2O, CO2, O3, N2O, CH4, SO2, NO2, Acrolein, benzene,
propene, toluene,
NH3, HNO3, H2O2, PH3, COF2, H2S
ethene
H2O, CO2, O3, N2O, CH4, SO2, NO2, Acrolein, benzene,
NH3, HNO3, H2O2, C2H2, PH3,
propene, toluene,
COF2, SF6, H2S
ethene
H2O, CO2, O3, N2O, CH4, SO2, NO2, Acrolein, propene,
toluene
NH3, HNO3, HF, HCl, HCN
H2O, CO2, O3, N2O, CH4, O2, NO,
Acrolein, benzene,
SO2, NH3, HNO3, OH, HCN, H2O2,
propene, toluene,
C2H2, PH3, H2S
ethene
H2O, CO2, O3, N2O, CH4, SO2, NO2, Acrolein, propene,
toluene
NH3, HNO3, HF, HCl, HCN
H2O, CO2, O3, N2O, CH4, SO2, OH,
29
2ν3
S2O
HSO
ν1
±100
2773
±100
1166.5
±100
ν2
382
±100
ν3
679
±100
ν1
2570±100
HCl, HBr, HI, N2
H2O, CO2, O3, N2O, CH4, NO2, OH,
HCl, HBr, H2CO
H2O, CO2, O3, N2O, CH4, SO2, NH3,
HNO3, HOCl, H2O2, C2H2, PH3,
COF2, H2S
H2O, CO2, O3, CH4, SO2, NH3,
HNO3, OH, HF, HCl, HBr, HI,
HOCl
H2O, CO2, O3, N2O, CH4, SO2, NO2,
NH3, HNO3, OH, HF, HCl, HI, ClO,
HOCl, HCN, C2H2, C2H6, PH3, COF2
H2O, CO2, O3, N2O, CH4, SO2, OH,
HCl, HBr,
Acrolein, propene,
toluene
Acrolein, ethene
Acrolein, propene,
toluene
Benzene, toluene
Toluene
Table 6. Gas components of aircraft jet measured by FTIR emission spectroscopy in ground-based
experiments [15]. Jet diameter is 50 см, temperature is 380оС.
Gas
CO2
H2O
CO
NO
NO2
N2O
SO2
H2CO
CH4
Spectral interval, cm- Interfering gases
1
2380-2400
1965-1985
2160-2180
1890-1910
1620-1640
2180-2200
1350-1370
2805-2825
3055-3075
(CO)
H2O, (CO2)
H2O, (CO, CO2)
H2O
CO, H2O, CO2
H2O, CO, (HC)
H2O, (CH4+…)
H2O
Concentration,
ppmV
1-4⋅104
2-6⋅104
10-3000
10-200
10-100
≤7
≤10
<15
<10
Detection limit,
ppmV
60
7000
20
90(>60)
60
10
340
50
150(>100)
Table 7. The values of νj and Σ j for spontaneous Raman scattering of different molecules.
Molecule
Cl2
CHCl3
F2
C6H6
SO2
CO2
N2O
C2H4
CO2
O2
C2H4
C2H3Cl
νj, cm-1
550
671
893
992
1151
1285
1285
1342
1388
1555
1623
1632
Σj
Molecule
CO
N2O
N2
H2S
HCl
CH4
C2H4
C6H6
NH3
H2O
HF
H2
2.2
2.6
0.3
11.0
4.4
0.7
2.0
2.8
1.1
1.0
1.6
1.6
30
νj, cm-1
2143
2224
2331
2611
2886
2914
3019
3070
3334
3652
3962
4156
Σj
0.9
0.5
1.1
6.8
3.1
8.3
6.4
14.5
5.8
4.5
1.3
3.8
NO
1877
0.3
Table 8. Typical concentrations of gases in the standard atmosphere at different altitudes H [43]
and in Boeing 707 wake [1]. L is the distance downstream the engine exit plane. Note: 1) HO2
concentration in the atmosphere was taken from [1]; 2) pressure and temperature for ambient
atmosphere at H=12.2 km were taken from [1]. (*) - fixed to ambient atmosphere value; (**) rough estimate as an average of the exit plane and ambient atmosphere values; (***) – corresponds
to 5% relative humidity - rough estimate for the plume axis with using [1] data.
Gas
N2
O2
H2O
CO2
N2O
CH4
CO
O3
H2CO
HCl
NH3
NO
SO2
H2O2
HCN
HNO3
NO2
HOCl
HI
HBr
HO2
OH
ClO
Mole fraction
Atmosphere
H=0 km
P=1 atm,
T=288.2 K
0.781
0.209
7.75⋅10-3
3.3⋅10-4
3.2⋅10-7
1.7⋅10-6
1.5⋅10-7
2.66⋅10-8
2.4⋅10-9
10-9
5⋅10-10
3⋅10-10
3⋅10-10
2⋅10-10
1.7⋅10-10
5⋅10-11
2.3⋅10-11
7.7⋅10-12
3⋅10-12
1.7⋅10-12
4.4⋅10-14
10-14
B707 wake at H=12.2 km
L=30.5 m,
Engine exit plane
P=0.1715 atm,
(L=0 m)
P=0.1715 atm,
T=338.9 K
T=604.7 K
0.789
0.785 (**)
0.159
0.184 (**)
-2
3.4⋅10
7.52⋅10-2 (***)
3.0⋅10-2
1.52⋅10-2 (**)
3.1⋅10-7 (*)
3.1⋅10-7 (*)
-6
1.66⋅10 (*)
1.66⋅10-6 (*)
2.5⋅10-5
1.25⋅10-5 (**)
3.1⋅10-7 (*)
3.1⋅10-7 (*)
3.39⋅10-11 (*)
3.39⋅10-11 (*)
4.36⋅10-11 (*)
4.36⋅10-11 (*)
-12
6.3⋅10 (*)
6.3⋅10-12 (*)
8.3⋅10-5
2.15⋅10-5
6.6⋅10-6
1.79⋅10-6
3.5⋅10-9
2.66⋅10-8
1.6⋅10-10 (*)
1.6⋅10-10 (*)
-12
6.2⋅10
1.4⋅10-7
9.1⋅10-6
2.51⋅10-6
5.01⋅10-12 (*)
5.01⋅10-12 (*)
3⋅10-12 (*)
3⋅10-12 (*)
1.7⋅10-12 (*)
1.7⋅10-12 (*)
-8
2.2⋅10
2.84⋅10-14
5⋅10-6
5.26⋅10-12
3.18⋅10-14 (*)
3.18⋅10-14 (*)
H=12.2 km
P=0.1715 atm,
T=217.3 K
0.781
0.209
1.91⋅10-5
3.3⋅10-4
3.1⋅10-7
1.66⋅10-6
7.81⋅10-8
3.1⋅10-7
3.39⋅10-11
4.36⋅10-11
6.3⋅10-12
3⋅10-10
5.6⋅10-11
1.95⋅10-11
1.6⋅10-10
2.41⋅10-10
3.15⋅10-11
5.01⋅10-12
3⋅10-12
1.7⋅10-12
3.6⋅10-13
4.94⋅10-14
3.18⋅10-14
31
Table 9. Cross sensitivities Qjk for analytical frequencies of СО2 lasers selected for detection of
SO2 gas in hot aircraft wake. Among the analytical frequencies (i.e., which satisfy the selection
criterion, see text) only the frequencies for SO2 detection were found. The frequencies (not
analytical) for detection of H2O, CO2, N2O and CH4 were chosen from all analytical ones for the
reason of maximum absorption cross section of given gas. The frequencies for detection of СО,
NO, NO2 and ОН were not considered because of very small cross sections. The conditions of
calculations see in Table. 8.
CO2
laser line
ν, cm-1
II626 R(62)
1099.6873
II626 R(30)
1084.6352
II828 R(54)
1114.3783
II628 R(47)
1101.3977
II828 R(54)
1114.3783
Interfering gas "k"
Gas "j"
to be
detected
H2O
1
0.569
7.72⋅10-5
6.14⋅10-4
0.902
CO2
1.44⋅10-5
1
1.87⋅10-3
9.62⋅10-6
0.200
N2O
5.16⋅10-4
5.12⋅10-6
1
1.69⋅10-2
3.54
CH4
4.34⋅10-2
0.732
2.44⋅10-4
1
9.91
SO2
1.46⋅10-4
1.45⋅10-6
0.282
4.77⋅10-3
1
H2O
CO2
N2O
CH4
SO2
Table 10. Minimum detectable concentrations c min
j , the estimate of total interference of other
gases
∑ Q jk
and absorption coefficients α at analytical frequencies of СО2 lasers in hot aircraft
j ≠k
wake (see also the heading in Table 9).
CO2 laser line
ν, cm-1
II626 R(62)
1099.6873
II626 R(30)
1084.6352
II828 R(54)
1114.3783
II628 R(47)
1101.3977
II828 R(54)
1114.3783
Detected
gas
c min
j , ppmV
∑ Q jk
α, km-1
j ≠k
H2O
50.287
1.4708
1.0155⋅102
CO2
20.349
0.20232
1.4773⋅102
N2O
59.289
3.5606
6.9845⋅10-2
CH4
218.29
10.685
10.747
SO2
16.733
0.28715
6.9845⋅10-2
32
Table 11. Cross sensitivities Q jk for "optimum" analytical frequencies (at which c min
are
j
reached) of fundamental band СО laser selected for detection of different gases in hot aircraft
wake. Among the analytical frequencies (i.e., which satisfy the selection criterion, see text) the
frequencies for detection of H2O, N2O, CH4, NO, SO2 and NO2 were found. The lines for detection
of CO2 and CO (not analytical) were chosen from all analytical ones for the reason of maximum
absorption cross section of given gas. The frequencies for detection of ОН were not considered
because of very small cross section. The conditions of calculations see in Table. 8.
CO laser
line
ν, cm-1
16-15 P(19)
1684.8358
10-9 P(9)
1874.4519
34-33 P(13)
1271.9622
7- 6 P(15)
1927.2960
32-31 P(19)
1299.6728
8-7 P(11)
1917.8613
28-27 P(23)
1379.9170
20-19 P(6)
1633.3132
6-5 P(20)
1931.6931
Gas
"j"
Interfering gas "k"
H2O
CO2
N2O
CO
CH4
NO
SO2
NO2
OH
H2O
1
0
2⋅10-9
0
2⋅10-6
3⋅10-7
0
5⋅10-5
2⋅10-29
CO2
0.08
1
9⋅10-2
10-5
4⋅10-3
40
0
3⋅10-6
2⋅10-22
N2O
10-4
2⋅10-7
1
0
9⋅10-4
0
10-7
7⋅10-5
3⋅10-38
CO
0.6
1
9⋅10-3
1
3⋅10-2
5⋅103
0
0
6⋅10-19
CH4
2⋅10-5
4⋅10-8
0.3
0
1
0
4⋅10-5
5⋅10-5
2⋅10-36
NO
10-3
1⋅10-5
2⋅10-6
4⋅10-8 3⋅10-8
1
0
0
9⋅10-25
SO2
3⋅10-4
5⋅10-7
3⋅10-6
0
3⋅10-4
0
1
2⋅10-5
7⋅10-38
NO2
4⋅10-5
0
2⋅10-3
0
10-4
4⋅10-10 0
1
4⋅10-32
OH
2⋅1015
4⋅1013
2⋅1013 3⋅1013 4⋅1014
9⋅1018
0
1
0
Table 12. Minimum detectable concentrations c min
j , the estimate of total interference of other
gases ∑ Q jk and absorption coefficients α at analytical frequencies of fundamental band СО
j ≠k
laser in hot aircraft wake (see also the heading in Table 8).
CO laser
line
ν, cm-1
16-15 P(19)
10-9 P(9)
34-33 P(13)
7- 6 P(15)
32-31 P(19)
8-7 P(11)
28-27 P(23)
20-19 P(6)
6-5 P(20)
1684.8358
1874.4519
1271.9622
1927.2960
1299.6728
1917.8613
1379.9170
1633.3132
1931.6931
Detected
gas
c min
j , ppmV
0.133
2.183⋅103
0.1728
1.235⋅104
1.0578
0.1921
0.3842
0.1404
4.85⋅1019
H2O
CO2
N2O
CO
CH4
NO
SO2
NO2
OH
33
∑ Q jk
α, km-1
j ≠k
5.014⋅10-5
38.63
1.137⋅10-3
5.169⋅103
0.3437
1.3526⋅10-3
6.7897⋅10-4
2.1129⋅10-3
8.85⋅1018
2.56⋅104
1.641
3.034
3.920
0.2411
67.15
4.4187
7.4747
1.6712
Table 13. Cross sensitivities Q jk for "optimum" analytical frequencies (at which c min
are
j
reached) of first -overtone СО laser selected for detection of different gases in hot aircraft wake.
Among the analytical frequencies (i.e., which satisfy the selection criterion, see text) the
frequencies for detection of H2O, CO2, N2O, CH4, NO, SO2, NO2 and OH were found. The
frequencies for CO detection were not considered because of very small cross section. The
conditions of calculations see in Table. 8.
CO laser
line
ν, cm-1
8- 6 P(21)
3853.9633
38-36 P(25)
2364.9292
35-33 P( 6)
2577.9590
22-20 P(25)
3131.7294
12-10 P( 8)
3707.6209
35-33 P(22)
2519.9057
27-25 P(25)
2888.9574
15-13 P( 8)
3554.7194
Gas
"j"
Interfering gas "k"
H2O
CO2
N2O
CH4
NO
SO2
NO2
OH
H2O
1
9⋅10-8
6⋅10-5
10-5
4⋅10-10
0
0
8⋅10-6
CO2
10-7
1
8⋅10-7
10-6
0
0
0
7⋅10-15
N2O
2⋅10-7
2⋅10-7
1
10-4
0
2⋅10-11
0
8⋅10-14
CH4
2⋅10-6
10-8
2⋅10-8
1
0
0
0
10-7
NO
0.1
0.5
8⋅10-4
7⋅10-8
1
0
0
4⋅10-3
SO2
9⋅10-6
7⋅10-7
4⋅10-2
5⋅10-3
0
1
0
10-11
NO2
10-6
0
0
5⋅10-2
0
0
1
3⋅10-7
OH
0.3
0.2
0
0
2⋅10-7
0
0
1
Table 14. Minimum detectable concentrations c min
j , the estimate of total interference of other
gases ∑ Q jk and absorption coefficients α at analytical frequencies of first-overtone СО laser in
j ≠k
hot aircraft wake (see also the heading in Table 13).
CO laser line
8-6
38-36
35-33
22-20
12-10
35-33
27-25
15-13
P(21)
P(25)
P( 6)
P(25)
P( 8)
P(22)
P(25)
P( 8)
ν, cm-1
3853.9633
2364.9292
2577.9590
3131.7294
3707.6209
2519.9057
2888.9574
3554.7194
Detected
gas
c min
j ,
ppmV
0.312
5.1⋅10-2
1.109
0.196
25.01
23.77
2.234
4.807
H2O
CO2
N2O
CH4
NO
SO2
NO2
OH
34
∑ Q jk
α, km-1
j ≠k
7.633⋅10-5
2.031⋅10-6
1.471⋅10-4
1.907⋅10-6
0.183
4.96⋅10-2
1.95⋅10-2
5.26⋅10-2
104
5.88⋅104
2.92⋅10-2
0.88
23.8
2.92⋅10-2
0.41
35.75